A fair reduction in gas prices would have been between 8.8 per cent and 10.3 per cent, the research indicated. Electricity prices should have been cut by 10 per cent.

This would have aligned prices with the wholesale cost of buying energy to supply to homes.

Which? also analysed the large price increases in late 2013, when bills rose by around 20 per cent, and found there was "no justification".

The analysis was based on commonly used methods for buying wholesale oil and gas and hedging against price fluctuations, it said.

In response, Danny Alexander, the Liberal Democrat Chief Secretary to the Treasury, said: "The Big Six should either come up with a credible justification for the slow pace of their current price falls or, better still, cut prices further and faster," he said.

Danny Alexander called on energy providers to cut bills further

Caroline Flint, Labour’s shadow energy secretary, said: “This research adds to the growing body of evidence showing that energy companies do not pass on reductions in wholesale costs as quickly or as fully as increases."

A spokesman for industry body Energy UK said: "The Which? calculations are based on very many assumptions.

"Each company allocates their costs differently and this makes it difficult to estimate wholesale costs and hedging strategies.

"Costs are coming down but, because energy companies buy ahead to fix prices and to plan with certainty, the gas and electricity we are using today has already been paid for."

Dr Richard Westoby, director of retail economics at SSE, one of the Big Six, said: “This analysis ignores many fundamental factors behind movements in household bills and suppliers’ costs, ranging from weather and consumption to green levies and network costs."